Low-temperature diffusion in inert-gas bombarded Al2O3

Previous work on diffusion in inert-gas bombarded Al₂O₃ has revealed the presence of four diffusion processes, of which two take place well below the temperatures for self-diffussion, one agrees with self-diffusion, and one occurs at temperatures well above those for self-diffusion. The present work serves to explore in greater detail the two low-temperature processes. It is shown that the first, which is found in -Al₂O₃, Al(OH)₃, and γ-Al₂O₃beginning at about 100° C, is consistent with a range of ΔH's of 28 to 50 kcal/mole. The mechanism of the process is hinted at by the fact that it overlaps the temperatures both for Al(OH)₃decomposition and for point-defect motion in -Al₂O₃; the correlation with point defects is believed, however, to be the more significant. The second process, which is found only in -Al₂O₃ beginning at 500–650° C, implies an essentially single ΔH lying between 69 and 79 kcal/mole. It was suggested previously by Matzke and Whitton on the basis of electron diffraction that the process could be attributed to the amorphous-crystalline transition of -Al₂O₃. Further aspects of low-temperature diffusion in Al₂O₃ were revealed by comparing autoradiographs of specimens of -A₂O₃which were bombarded to various doses and then either heated to 850° C or immersed in unheated 12N NaOH. Thus regions exposed to a high dose and which would be expected to be amorphous, had an increased sticking factor, a greater tendency to lose gas during heating, and an enhanced chemical reactivity.     

Al2O3样品制备技术及^26Al的AMS测量

报道了用于加速器质谱计^26Al分析的Al2O3的制备流程及^26Al的测量过程。制备的空白样品^26Al/^27Al比值＜10^－13,显示出同量异位素^26Mg干扰小。制备流程是成功的。     

基于铌酸银的耐高温放射性碘吸附剂的制备及性能

通过机械混合法制备了一种基于铌酸银(AgNbO_3)的耐高温放射性碘吸附剂(AgNbO_3/Al_2O_3)。和常规载银吸附剂(Ag/Al_2O_3)相比,AgNbO_3/Al_2O_3吸附剂的吸附性能更为稳定;特别是在650℃以上时,其对放射性碘的去污因子远高于常规载银吸附剂。表征测试结果表明,该吸附剂结构稳定性良好,可耐受较长时间的高温。热重测试和高温脱附试验等结果表明,碘化银在AgNbO_3/Al_2O_3吸附剂表面稳定性的提高是该吸附剂在高温时吸附性能更佳的主要原因,其将来有望用于核事故中的应急处置。     

Scattering of light molecules from thin Al2O3 films

Molecular oxygen and hydrogen ions were scattered at grazing incidence from various thin Al₂O₃ films. The energy of incident particles was varied from 390 to 1000 eV. For scattered positive oxygen ions, negative ion fractions of up to 17% were recorded. For scattered positive hydrogen ions, the negative ion fractions reached up to 2%. These findings qualify thin films of Al₂O₃ as possible candidates for use as charge state conversion surfaces in neutral particle sensing instruments, which will work in space.     

Ion guiding through capillaries in uncoated Al2O3 membranes

We made an experimental study on ion guiding through capillaries in uncoated Al₂O₃ membranes using a variety of ions such as O¹⁺, O³⁺, and O⁶⁺. The incident energy was varied within the range of 30-150 keV. The results were compared with others using coated PET and Al₂O₃ capillary membranes as well as with the so-called scaling law discovered by Stolterfoht and his co-workers. Good agreement of our results with the scaling law was found. However, our membranes showed extraordinarily strong guiding ability. The reason lies in that our membranes were uncoated. A slower charge drift speed along the insulating capillary wall and a much larger equilibrium charge Q_∞ seems to exist in our experiment.     

Enhanced oxygen vacancy aggregation and colloid production in Al2O3

Optical luminescence and absorption measurements on Al₂O₃ (sapphire) electron irradiated at temperatures between 200 and 270°C with and without an applied electric field have been used to identify an oxygen vacancy aggregation process leading to the formation of aluminium colloids. This process, which is only observed when an electric field is applied during irradiation, occurs within the volume and is considered as a possible precursor for RIED. The results help to clarify the observed similarity between RIED and colloid production, and help to provide an explanation for the observation of gamma alumina in sapphire which has suffered RIED.     

添加Al2O3和SiO2的大晶粒UO2芯块制备研究

研究了Al2O3和SiO2添加剂对UO2芯块晶粒尺寸的影响.结果表明加入少量的Al2O3和SiO2,可有效促进烧结过程中UO2芯块的晶粒度长大,过量加入则会阻碍烧结过程中UO2芯块的致密化;在添加量一定的情况下,添加不同比例的Al2O3和SiO2,对芯块晶粒尺寸有较大影响,只添加SiO2,对芯块晶粒尺寸影响不大,Al2O3添加量增加,芯块晶粒尺寸随之增加;添加Al2O3和SiO2促进UO2芯块晶粒长大的机制是在烧结期间发生了液相烧结.     

Plasma-Assisted ALD of an Al2O3 Permeation Barrier Layer on Plastic

Atomic layer deposition(ALD) technique is used in the preparation of organic/inorganic layers,which requires uniform surfaces with their thickness down to several nanometers.For film with such thickness,the growth mode defined as the arrangement of clusters on the surface during the growth is of significance.In this work,Al 2 O 3 thin film was deposited on various interfacial species of pre-treated polyethylene terephthalate(PET,12 μm) by plasma assisted atomic layer deposition(PA-ALD),where trimethyl aluminium was used as the Al precursor and O 2 as the oxygen source.The interfacial species,-NH 3,-OH,and-COOH as well as SiCHO(derived from monomer of HMDSO plasma),were grafted previously by plasma and chemical treatments.The growth mode of PA-ALD Al 2 O 3 was then investigated in detail by combining results from in-situ diagnosis of spectroscopic ellipsometry(SE) and ex-situ characterization of as-deposited layers from the morphologies scanned by atomic force microscopy(AFM).In addition,the oxygen transmission rates(OTR) of the original and treated plastic films were measured.The possible reasons for the dependence of the OTR values on the surface species were explored.     

Investigation of Stable Al2O3 Scale on Fe-Al Aluminized Coating for Tritium Barrier

It is well known that α-Al₂O₃ phase has stablility performance, high permeation reduction factor and good resistance performance in liquid LiPb, which is considered as the reference tritium barrier coating in future fusion reactor. In order to study the formation mechanism of stable α-Al₂O₃ scales on fusion structure material, the oxidation behavior of Fe-Al aluminized coating on China Low Activated Martensitic （CLAM） steel was investigated under the oxygen partial pressure from 1 Pa to 20 kPa at the temperature of 940-980 ℃. The Al₂O₃ scales were analyzed by thermogravimetric analysis meter, grazing angle X-ray diffractometer, glow discharge spectrometer, focused ion beam and transmission electron microscope. A single continuous Al₂O₃ scales with the maximum thickness of about 2 000 nm was formed on the diffusion Fe-Al aluminized layer. Thermogravimetric analysis results show that the higher oxidation rate constant is achieved while increasing the oxygen partial pressure, and then oxidation rate constant decreases. The phase transformation of Al₂O₃ scales on the surface of Fe-Al aluminized coating was studied during different oxidation time ranges from 3 min to 180 min. The metastable γ-Al₂O₃ and α-（Al0.948Cr0.052）₂O₃ phases is formed in the earlier oxidation process and finally transformed to stable α-Al₂O₃ phase. The features of the transient α-（Al0.948Cr0.052）₂O₃（113） and α-Al₂O₃（113） were detected by GXRD and then confirmed by focused ion beam and transmission electron microscope.     

Argon irradiation damage at a Cu/Al2O3 interface for different alumina structures

The evolution of damages at a Cu/Al₂O₃ device interface after Ar⁺ irradiation, depending on alumina structure, and the effect of surface roughness on sputtering have been studied. A polycrystalline Cu/Al₂O₃ bilayer and polycrystalline Cu on amorphous alumina were irradiated with 400 keV Ar⁺ ion beam at doses ranging from 5 × 10¹⁶ to 10¹⁷ Ar⁺/cm² at room temperature. The copper layer thicknesses were between 100 and 200 nm. RBS analysis was used to characterize the interface modification and to deduce the sputtering yield of copper. The SEM technique was used to control the surface topography. A RBS computer simulation program was used to reproduce experimental spectra and to follow the concentration profile evolutions of different elements before and after ion irradiation. A modified TRIM calculation program which takes into account the sputtering yield evolution as well as the concentration variation versus dose gives a satisfactory reproduction of the experimental argon distribution. The surface roughness effect on sputtering and the alumina structure influence at the interface on mixing mechanisms are discussed.     

Diffuse reflectance and X-ray photoelectron spectroscopy of uranium in ZrO2 and Y2Ti2O7

Diffuse reflectance measurements were made over the wavenumber range of 4000-20,000 cm⁻¹ at room temperature on monoclinic and stabilised ZrO₂, together with Y₂Ti₂O₇ having the pyrochlore structure, all of which were doped with U and sintered in various atmospheres. X-ray photoelectron spectroscopy measurements were also carried out on selected samples. In monoclinic and stabilised zirconia, U exhibited valence states of +4 and/or +5, depending on the sintering atmosphere and the presence of appropriate charge compensators. Using both diffuse reflectance and X-ray photoelectron spectroscopy, U was also observed as mainly U⁴⁺ and/or U⁵⁺ in U-doped Y₂Ti₂O₇ sintered at 1400 °C in air or Ar, although a small amount of U⁶⁺ also appeared to be present in some U-doped Y₂Ti₂O₇ samples heated in air.     

Absorption and photoluminescence study of Al2O3 single crystal irradiated with fast neutrons

Colour centers formation in Al₂O₃ by reactor neutrons were investigated by optical measurements (absorption and photoluminescence). The irradiation’s were performed at 40 °C, up to fast neutron (E_n > 1.2 MeV) fluence of 1.4 × 10¹⁸ n cm⁻². After irradiation the coloration of the sample increases with the neutron fluence and absorption band at about 203, 255, 300, 357 and 450 nm appear in the UV-visible spectrum. The evolution of each absorption bands as a function of fluence and annealing temperature is presented and discussed. The results indicate that at higher fluence and above 350 °C the F⁺ center starts to aggregate to F center clusters (F₂, F₂⁺ and ). These aggregates disappear completely above 650 °C whereas the F and F⁺ centers persist even after annealing at 900 °C. It is clear also from the results that the absorption band at 300 nm is due to the contribution of both F₂ center and interstitial ions.     

HfO2 and Ta2O5 as grain growth inhibitors in Eu2O3

Because of the high neutron capture cross section for five consecutive europium isotopes, Eu₂O₃ is of interest as a control material for nuclear reactors. A tendency toward excessive grain growth degrades its mechanical properties. Small amounts of HfO₂ and Ta₂O₅ were added to the Eu₂O₃ in attempts to suppress this grain growth. Three at % substitution of Hf for     

Phase changes of the ZrO2-12wt%Y2O3 film with Ar+ irradiation

The zirconia containing 12wt%Y₂O₃ thin films deposited by r.f. magnetron sputtering at 25℃ or 400℃, and then bombarded with Ar⁺ beam at room temperature were characterized with XRD before and after Ar⁺ bombardment. It is found that a series of phases formation and transformation happened, among them the mostimportant event is that T' phase appeared after Ar⁺ irradiation andthe content of the T' phase increased with the increase of Ar⁺ iondoses from 5×10¹⁵ to 6×10¹⁶ ions cm^-2.     

Thermal-hydraulic characteristics of a single-phase natural circulation loop with water and Al2O3 nanofluids

A natural circulation system operates on the basis of natural laws like gravity and buoyancy. Although natural circulation is a benign gift of nature for applications to several heat removal systems due to their simplicity in design, elimination of hazards related to pumps, better flow distribution, cost reduction, etc. however, the potential threat of flow instabilities still eludes for its wide applications. Although addition of local losses (orificing) may suppress instabilities, however, it is accompanied by significant flow reduction which is detrimental to the natural circulation heat removal capability. In this paper, we have demonstrated experimentally, with nanofluids, not only the flow instabilities are suppressed but also the natural circulation flow rate is enhanced. The increase in steady natural circulation flow rate due to addition of nanoparticles is found to be a function of its concentration in water. The flow instabilities are found to occur with water alone only during a sudden power addition from cold condition, step increase in power and step decrease in power (step back conditions). With a small concentration of Al₂O₃ nanofluids, these instabilities are found to be suppressed significantly.     

Electron Cyclotron Resonance Plasma-Assisted Atomic Layer Deposition of Amorphous Al2O3 Thin Films

Without extra heating, Al2O3 thin films were deposited on a hydrogen-terminated Si substrate etched in hydrofluoric acid by using a self-built electron cyclotron resonance (ECR) plasma-assisted atomic layer deposition (ALD) device with Al(CH3)3 (trimethylaluminum; TMA) and O2 used as precursor and oxidant, respectively. During the deposition process, Ar was introduced as a carrier and purging gas. The chemical composition and microstructure of the as-deposited Al2O3 films were characterized by using X-ray diffraction (XRD), an X-ray photoelectric spectroscope (XPS), a scanning electron microscope (SEM), an atomic force microscope (AFM) and a high-resolution transmission electron microscope (HRTEM). It achieved a growth rate of 0.24 nm/cycle, which is much higher than that deposited by thermal ALD. It was found that the smooth surface thin film was amorphous alumina, and an interfacial layer formed with a thickness of ca. 2 nm was observed between the Al2O3 film and substrate Si by HRTEM. We conclude that ECR plasma-assisted ALD can grow Al2O3 films with an excellent quality at a high growth rate at ambient temperature.     

氧化铝纳米流体临界热流密度机理模型研究——物理模型

针对现有纳米流体临界热流密度（CHF）模型存在的不足,基于气泡力平衡分析方法,考虑接触角和毛细现象带来的影响,构建了针对氧化铝纳米流体CHF的机理模型。结果表明:模型可模拟CHF随纳米流体浓度（c_NF）变化的规律,随着c_NF增加,CHF开始增加;但增加至某一浓度之后,CHF不再增加而维持恒定值;模型表明CHF与纳米微粒直径（d₀）无关,这与已有实验结果吻合;随着接触角或倾斜角增加,模型计算得到的CHF减小。      

Cavitation erosion and jet impingement erosion mechanism of cold sprayed Ni–Al2O3 coating

A composite coating was deposited on Inconel 600 substrate by cold spray method using pure Ni powder (60 wt.%) blended with α-Al₂O₃ (40 wt.%) as feedstock. It is expected to be applied to repair the bellows eroded by the liquid droplet erosion (LDE). Microstructure of the coating was observed using optical microscope (OM) and scanning electron microscopy (SEM). Microhardness of the coating was determined by Vickers hardness tester. Cavitation erosion (CE) experiments were carried out in the distilled water. Jet impingement erosion (JIE) experiments were performed in slurry containing 1 wt.% quartz particle with the flow velocity of 15 m/s at impingement angles of 30°, 60° and 90°, respectively. Cumulative mass loss vs. testing time was used to evaluate the erosion rate of the coating. The erosion mechanism was analyzed by OM, SEM, X-ray diffraction (XRD) and the microhardness measurement. The results show that the composite coating has compact microstructure and relatively high hardness. The resistance to CE of the coating is not as good as that of Inconel 600 substrate due to the weak bonds of the Al₂O₃ particles. However, the results of the JIE test indicate that the slurry erosion resistance of the coating is better than that of Inconel 600 at the impact angles of 30° and 60°, but not at the normal impact angle.     

Enthalpy measurements of La2Te3O9 and La2Te4O11

Enthalpy increment measurements on La₂Te₃O₉(s) and La₂Te₄O₁₁(s) were carried out using a Calvet micro-calorimeter. The enthalpy values were analyzed using the non-linear curve fitting method. The dependence of enthalpy increments with temperature was given as: H°(T) − H°(298.15 K) (J mol⁻¹) = 360.70T + 0.00409T² + 133.568 × 10⁵/T − 149 923 (373 ? T (K) ? 936) for La₂Te₃O₉ and H°(T) − H°(298.15 K) (J mol⁻¹) = 331.927T + 0.0549T² + 29.3623 × 10⁵/T − 114 587 (373 ? T (K) ? 936) for La₂Te₄O₁₁.